Pull static assets from a configurable CDN

Added some logic in our grunt file that allows us to specify a CDN.

Note that the default configured CDN is pulling assets from production. If you want to test this, you could do the following:
 * Add a `static.oae.com` in your /etc/hosts file
 * Replace the cdn url with `static.oae.com`
This gives an error `Object #<Object> has no method 'set'` I think it should be something like `grunt.config.set('replace.url', url);`